1. Field of the Invention
The invention relates generally to a method of manufacturing semiconductor devices and, more particularly, relates to a technique in which an element isolation layer is formed in a semiconductor substrate by ion implantation.
2. Description of the Background Art
Performance of semiconductor devices has been remarkably improved in recent years and rapid progress is being made in increasing integration density and the operation speed and decreasing power consumption. With increased integration density, a junction capacitance attendant on a pn junction for element isolation becomes a parasitic capacitance and causes disadvantageous decrease in the operation of circuit elements. In order to form a faster semiconductor device with reduced power consumption, it is an important subject to reduce this parasitic capacitance.
For example, some parasitic capacitances are generated between a collector and a substrate in a bipolar device while others are generated between source/drain and a substrate in a MOS (Metal Oxide Semiconductor) device.
A conventional method for reducing the parasitic capacitance is that a silicon substrate is provided with a region where oxygen ions of a high concentration are to be implanted, and a surface silicon layer and substrate silicon are separated from each other using a buried oxide film formed by high temperature anneal as an insulating layer. This method is normally called "SIMOX (Separation by Implanted Oxygen)".
A description will now be made of one example of a manufacturing process of a semiconductor device to which the conventional SIMOX method above is applied, with reference to FIGS. 1A to 1G. Firstly, oxygen ions are implanted on the entire main surface of a semiconductor substrate 1 formed of silicon single crystal or the like shown in FIG. 1A with implantation energy of 180 to 200 KeV and the dose of 1.8 to 2.0.times.10.sup.18 /cm.sup.2 as shown in FIG. 1B, so that an oxide silicon layer 2 is formed in a predetermined depth within the semiconductor substrate 1. The semiconductor substrate 1 is separated into an upper silicon layer 1a and a lower silicon layer 1b by this oxide silicon layer 2.
Referring now to FIG. 1C, a silicon oxide film is formed over the entire main surface of the semiconductor substrate 1 with a thermal oxidation method or a CVD method. Thereafter, furthermore, referring to FIG. 1D, a silicon nitride film 4 is formed over the entire surface of the silicon oxide film 3 by the CVD method.
Then, the silicon nitride film 4 is selectively removed to be patterned by a photolithography technique. After that, a mask 5 shown in FIG. 1E is patterned by selectively removing the exposed silicon oxide film 3 by a dry etching method such as reactive ion etching, using the patterned silicon nitride film 4 as a mask.
Then, by oxidizing the semiconductor substrate 1 in an atmosphere of high temperature oxidation, the exposed portion of the upper silicon layer la of the semiconductor substrate 1 is oxidized and a thick silicon oxide film 6 is formed. The silicon oxide film 6 is oxidized until it is in contact with the oxide silicon layer 2, as shown in FIG. 1F.
Subsequently, after removing, with phosphoric acid or the like, the silicon nitride film 4 used as a mask in the oxidation process for forming the silicon oxide film 6, the semiconductor substrate 1 is treated in an acid solution to remove the silicon oxide film 3, so that the upper silicon layer 1a except the region where the silicon oxide film 6 is formed is exposed (FIG. 1G).
As stated above, in accordance with this conventional method, the upper silicon layer 1a which is an active region is surrounded by the silicon oxide film 6 and the oxide silicon layer 2, so that a so-called electrically isolated complete element isolation structure can be obtained.
According to the conventional method above, however, a defect of the crystal is caused in the upper silicon layer 1a to be an active region and the element characteristics become deteriorated since the oxide silicon layer 2 is formed by implanting oxygen ions from the entire main surface of the semiconductor substrate 1.
A manufacturing method for solving the problem of the conventional method above is described in Japanese Patent Laying-Open (KOKAI) No. 61-185950. A description of the manufacturing process described herein will be made with reference to FIGS. 2A to 2E.
In the process of manufacturing a semiconductor element described herein, firstly, as shown in FIG. 2A, a mask 15 of a predetermined pattern including a silicon oxide film 13 and a silicon nitride film 14 is formed in the same way as of the mask 5 shown in FIG. 1E. Subsequently, referring to FIG. 2B, oxygen ions are implanted over the entire main surface of a semiconductor substrate 11 with a predetermined ion implantation energy and dose and at a predetermined angle of inclination, so that an ion implantation layer 12a is formed to be discontinuous in a predetermined position of a predetermined depth within the semiconductor substrate 11. Furthermore, referring to FIG. 2C, oxygen ions are implanted over the main surface of the semiconductor substrate 11 at an angle symmetrical to that in the case of FIG. 2B and with the same ion implantation energy and dose to form a continuous ion implantation layer 12b. After that, a heat treatment at 1100.degree. C. or above is carried out to form a buried insulating layer 12 formed of oxide silicon, so that the semiconductor substrate 11 is separated into an upper silicon layer 11a and a lower silicon layer 11b (FIG. 2D).
Referring to FIG. 2E, a silicon oxide film 16 is formed and an active region 17 in the upper silicon layer 11a is completely electrically isolated by selectively oxidizing the upper silicon layer 11a in an atmosphere of oxidation.
In the manufacturing process, the oxygen ions are not directly implanted into the upper silicon layer 11a of the active region, so that degradation of the properties of crystal in the portion is prevented.
The method disclosed in the publication above can prevent degradation of the properties of crystal of the semiconductor substrate 11 to be the active region 17 immediately below the mask and prevent the residual oxygen ions; however, it requires an additional process of selective oxidation for further forming the silicon oxide film 16 on the side surface after forming the buried insulating layer 12, resulting in increase of the number of manufacturing processes and reduction of the productivity.